The present invention relates to a display which uses electrochromic nanoparticles.
Electrophoretic displays have been the subject of intense research and development for a number of years. Such displays use a display medium comprising a plurality of electrically charged particles suspended in a fluid. Electrodes are provided adjacent the display medium so that the charged particles can be moved through the fluid by applying an electric field to the medium. In one type of such electrophoretic display, the medium comprises a single type of particle having one optical characteristic in a fluid which has a different optical characteristic. In a second type of such electrophoretic display, the medium contains two different types of particles differing in at least one optical characteristic and in electrophoretic mobility, the particles may or may not bear charges of opposite polarity. The optical characteristic which is varied is typically color visible to the human eye, but may, alternatively or in addition, be any one of more of reflectivity, retroreflectivity, luminescence, fluorescence, phosphorescence (in the case of displays intended for machine reading) color in the broader sense of meaning a difference in absorption or reflectance at non-visible wavelengths.
Electrophoretic displays can be divided into two main types, namely unencapsulated and encapsulated displays. In an unencapsulated electrophoretic display, the electrophoretic medium is present as a bulk liquid, typically in the form of a flat film of the liquid present between two parallel, spaced electrodes. Such unencapsulated displays typically have problems with their long-term image quality which have prevented their widespread usage. For example, particles that make up such electrophoretic displays tend to cluster and settle, resulting in inadequate service-life for these displays.
An encapsulated, electrophoretic display differs from an unencapsulated display in that the particle-containing fluid is not present as a bulk liquid but instead is confined within the walls of a large number of small capsules. Encapsulated displays typically do not suffer from the clustering and settling failure mode of traditional electrophoretic devices and provides further advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates.
For further details regarding encapsulated electrophoretic displays, the reader is referred to U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584; 6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773; 6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,249,721; 6,252,564; 6,262,706; and 6,262,833, and International Applications Publication Nos. WO 97/04398; WO 98/03896; WO 98/19208; WO 98/41898; WO 98/41899; WO 99/10769; WO 99/10768; WO 99/10767; WO 99/53373; WO 99/56171; WO 99/59101; WO 99/47970; WO 00/03349; WO 00/03291; WO 99/67678; WO 00/05704; WO 99/53371; WO 00/20921; WO 00/20922; WO 00/20923; WO 00/36465; WO 00/38000; WO 00/38001; WO 00/36560; WO 00/20922; WO 00/36666; WO 00/59625; WO 00/67110; WO 00/67327; WO 01/02899; WO 01/07961; WO 01/08241; WO 01/08242; WO 01/17029; WO 01/17040; and WO 01/17041. The entire disclosures of all these patents and published applications, all of which are in the name of, or assigned to, the Massachusetts Institute of Technology or E Ink Corporation, are herein incorporated by reference.
Prior art electrophoretic displays use particles, which, while small (typically about 0.25 to 2 Î¼ m), are sufficiently large that they have essentially the bulk properties of the material from which they are formed. The particles keep the same optical properties during the time they are present in the electrophoretic display; the appearance of the display is changed by moving the particles within the suspending fluid using an appropriate electrical field.
Nanoparticles have diameters from about 1 to about 100 nanometers. Particles in this size range do not generally scatter incident light efficiently unless they are concentrated. The aforementioned U.S. Patent No. 6,323,989 describes nanoparticle-based reflective displays where the display varies from transparent or translucent to opaque depending on whether the nanoparticles are dispersed or aggregated.
Displays are also known based upon electroluminescent materials. Such materials emit light after being excited by the passage of electric current through the materials. The passage of the electric current raises electrons within the electroluminescent material to excited states, from which the electrons return to their ground states with emission of radiation. Accordingly, electroluminescent displays are emissive and emit light only for so long as the current is passed. This behavior is in contrast to the electrophoretic and nanoparticle-based displays previously described which, because they rely only upon the movement or aggregation of particles are bistable in that once the display has been driven to a desired state, that state will persist for a substantial period without further supply of energy to the display, i.e., such electrophoretic and nanoparticle-based displays are passive, in contrast to the emissive electroluminescent displays.
Electrochromic displays are also well known. Electrochromic materials are those whose color changes with oxidation state, that is by addition of electrons to, or withdrawal of electrons from, molecular orbitals. Note that, in contrast to electroluminescent materials, the optical characteristics of electrochromic materials remain constant so long as the oxidation state of the materials remains the same, so that a display based upon electrochromic materials is passive, and once the display has been driven to a desired state, that state will persist for a substantial period without further supply of energy to the display.
Two types of electrochromic display are common, namely metal-oxide electrochromic displays and molecular electrochromic displays. Electrochromic metal oxides change optical properties in response to the injection of electron charge (anodic) or the withdrawal of electron charge (cathodic); see, for example, Zum Felde, U., et al., J. Phys. Chem. B2000, 104, 9388. Various models have been formulated to explain the electrochromic mechanism. Electrochromic displays consist of up to seven layers of materials, and rely upon transport of hydrogen or lithium ions from an ion storage layer, through an ion-conducting layer, and injection of these ions into an electrochromic layer. The electrochromic layer is typically tungsten oxide (WO3). The presence of the ions in the electrochromic layer changes its optical properties, causing it to absorb visible light. The large-scale result is that the display darkens. The ion-conducting, ion storage and electrochromic layers are sandwiched between two layers of a transparent conducting oxide material. To protect these five layers, they are further sandwiched between two layers of glass. All of the layers, of course, are transparent to visible light. See www.nrel.gov/buildings/windows/how.html; Zhang, J. Ci, et al., “Chromic mechanism in amorphous WO3 films”, J. Electrochem. Soc., 1997, 144(6), 2022; and www.schottdonnelly.com. Such metal-oxide electrochromic displays are relatively slow because of the time for ion diffusion.
Molecular electrochromic materials change optical properties in response to the injection of electron charge (reduction) or the withdrawal of electron charge (oxidation); see, for example, Tian, J., et al., “Electroluminescent properties of self-assembled polymer thin films”, Adv. Mater 1995, 7, 395–398; and on the University of Florida website:
“Electron rich electrically conducting, redox electroactive, and electrochromic polymers are especially interesting due to their stability in the conducting state and ability to be repeatedly switched between charged and neutral states many times with large changes in properties (such as color). The Reynolds Research Group is developing a family of derivatized poly(3,4-alkylenedioxythiophene)s (PXDOTs) which provide a number of outstanding properties. As electrochromic polymers, these materials switch from a dark opaque blue in their reduced form to a highly transmissive light blue in their oxidized form. We synthesize these polymers with a combination of transition metal mediated solution and electrochemical polymerizations. The polymer's properties are varied by changing either the nature of the pendant group or the size of the alkylenedioxy ring. We find the PXDOTs to exhibit quite high electrochromic contrast ratios as desired in switchable mirror, window, display, and other devices. They also switch quite rapidly with nearly complete color changes being attained in 0.25 to 0.5 seconds. The more highly substituted polymers exhibit the largest electrochromic contrasts and response times”.
One advantage of electrochromic polymers over metal oxides is their higher speed. The polymers are usually coated directly on to an electrode. One disadvantage of electrochromic polymers is that optical densities tend to be lower than those of electrochromic metal oxide.
It is also known that certain nanoparticles are electrochromic; see Wang, C., et al., “Electrochromic nanocrystal quantum dots”, Science, 2001, 291, 2390–2392. This paper states that the optical properties of semiconducting cadmium selenide nanoparticles are changed by reduction of the nanoparticles at an electrode. Subsequent oxidation returned the particles to their original optical state. The injection of electrons into the quantum-confined states of the nanoparticle led to three electrochromic responses: the creation of a size-dependent mid-infrared absorption, a bleaching of the visible absorption, and a quenching of the luminescence. The bleaching of the visible absorption and the quenching of the luminescence changed the color of the particle. If the bleaching and quenching are sufficiently complete, the particle is transparent in the visible.
The present inventors have realized that it is possible to construct displays using electrochromic nanoparticles and that such displays offer substantial advantages over the prior art electrochromic displays described above.